1. Field of the Invention
The present invention is directed to an improved turbine power plant control system and, more particularly, to a system that reduces overall start up time for the turbine by overlapping a steam chest warming operation, speeding up shaft critical run through, and shortening vibration run up.
2. Description of the Related Art
Turbine power systems typically include a high pressure turbine section where the steam is introduced from the steam generator. The steam from the high pressure turbine section after being reheated is introduced into a reheat turbine section, which in the case of a fossil fired steam generating system is commonly termed the intermediate pressure turbine section and then into a low pressure turbine section before exhausting to the condenser. A rotor having an axial bore passes centrally through the turbine casing and rotation of the rotor is achieved by passage of steam over blades affixed to the rotor revolving in the casing. The generator which is affixed to the rotor may be cooled by hydrogen gas. A steam chest on both the right and left sides of the turbine includes throttle valves and governor valves for controlling the steam applied to the high pressure turbine. The metal mass of the steam chest must be brought to operating temperature before the full pressure of the supplied steam can be admitted into the steam chest.
In a typical startup, the pilot valves within the throttle valves admit the steam flow through the steam chest into the turbine to control the turbine speed. All the governor valves are wide open since the amount of the steam flow is far below their controllable range. At approximately ninety percent of the rated speed, the governor valves are closed downward until a speed drop occurs, indicating the governor valves have taken control or the steam flow, then the pilot valves and throttle valves are ramped upward all the way to wide open, thereby transferring control to the governor valves
In a typical or conventional warm start, after a weekend shutdown, the inlet steam to the turbine is colder than the rotor surface and the turbo generator unit will go through a period of forced cooling to roll up to the valve transfer speed. The transfer is, in most cases delayed because of a cold steam chest inner metal temperature, thus prolonging the forced cooling of the rotor and creating high thermal stresses that, in turn, affect later acceleration. The warming of the steam chest is rather slow because very little steam is emitted through the throttle pilot valves. During a warm start, the rotor reaches the ninety percent rated speed point before the steam chest reaches operating temperature, requiring that the throttle valves continue to control until the steam chest reaches the saturation temperature at the existing pressure. This is what prolongs rotor cooling and increases rotor stress.
What is needed is a system which will lower or close the governor valves to maintain rolling speed and open the throttle valve pilot valve to the full stroke, thereby overlapping the throttle valve and governor valve control mode and partially pressurizing the steam chest. This will allow more steam to flow through the steam chest and the warming process can be shortened significantly while still maintaining rolling speed.
During the operation for controlling the power plant from cold, warm or hot start up to application of a full load the conventional automatic control system provides for accelerating the turbine from zero speed through heat soak speed to synchronous speed in accordance with the real time thermal stresses in the system. During such control, the system can vary the rate of acceleration by either stopping acceleration altogether, holding it constant, increasing it or decreasing it. Conventional control operations allow the rate of acceleration to be reduced below a value at which stress is not a problem even though this is not necessary, thereby resulting in further inefficiencies in the start up operation. What is needed is a system that will only fall to a minimum rate of acceleration for the unit to run through shaft critical and blade resonant speed ranges. This minimum will keep rotor stress within an allowable range. Such a system which will not restrict the rate of increase because of time delay requirements between speed changes and thereby miss speed windows.
In conventional systems, during vibration run down, run through and run up operations a five minute counter is maintained to track the trending of vibration measurements. The system is looking for a monotonous decreasing trend. Any spike or blip in a trend reading that is counter to the monotonous decreasing trend resets the counter and restarts the five minute count. What is needed is a system which determines the rate of trending and prevents the resetting of the counter by a single spike or blip in the readings.